In the field of fluid pumps generally, compressed air continues to be a popular means for driving the reciprocating motion of a pump piston. Usually, this involves a piston tube in which the piston is moved back and forth in response to alternating streams of inrushing compressed air. In particular, air is forced to flow into the tube first on one side of the piston thus driving the piston in one direction. Then, at a preselected point, the first flow of air is halted, and a second flow is initiated on the opposite side of the piston which forces the piston to move back in the direction from whence the piston originally came. Then, when the piston is moved sufficiently in this second direction, the second flow is halted and a third flow is commenced which is identical in origin and action to the first flow to push the piston the same direction as the first flow did. As above, the third flow is ultimately halted and a fourth, identical to the second, is started. This cycle is then repeated over and over to reciprocate the piston and thereby drive the desired quantity of fluid, such as a lubricant, through the system to its preselected delivery point. A foot valve may be used to effectuate the actual movement of the fluid through the pump. Such a foot valve is connected to the piston rod and thus moves therewith in response to the alternating air flows in the piston tube. A typical foot valve assemblage is described in the preferred embodiment below.
A common concern with respect to pneumatically operated pumps has involved the control of the alternating air flows in and out of the piston tube to drive the piston back and forth. Many prior art devices have made use of sliding members to effect the desired control of air flow into the respective chambers on either side of the piston. Examples of such prior art devices include those in current production by the Alemite Corporation, Charlotte, N.C. Further, increasingly complex sliding (and the like) controls are disclosed in Hillis, U.S. Pat. No. 2,143,733; Scholle, U.S. Pat. No. 4,123,204; Dooley et al., U.S. Pat. Nos. 4,305,531, and 4,438,872; Stachowiak, U.S. Pat. No. 4,878,815; and Miller et al., U.S. Pat. No. 5,044,254.
In practice, such sliding control members suffer large amounts of friction due to the sliding contact and the requisite close tolerances necessary to seal the respective chambers. This friction causes a great deal of wear and tear on the sliding control members which, as a result, have relatively short use lives. Thus, there remains a need for means and methods to alternately seal and unseal the pneumatic chambers of such pumps in a fashion which will provide for extended part lives. It is toward this desideratum that the present invention is primarily directed.